This invention relates to the field of solar cells for converting solar energy into electrical energy. More particularly, this invention relates to solar cells having front and back electrodes.
Solar cells comprising semiconductor devices employing the photovoltaic effect for converting solar energy into electrical energy have long been known. A typical solar cell comprises a semiconductive substrate provided with metal electrodes on one or more surfaces of the substrate, the electrodes being electrically connected to the substrate at the surface. The vast majority of solar cell designs incorporate a p-n junction within the semiconductive substrate for separating the electrons and holes generated within the body of the semiconductive substrate in response to the incidence of solar radiation. The metal electrodes are characterized as either the p-type electrode, used to collect the holes or the n-type electrode, used to collect electrons. In a single-sided solar cell, both types of electrodes are located on a single major surface of the substrate. In a double-sided solar cell, the electrodes of one type are located on one major surface of the substrate, while the electrodes of the other type are located on the other major surface of the substrate. A common double-sided cell configuration provides a single common electrode on the back surface of the cell and a plurality of electrodes on the front surface of the cell. This plurality of electrodes is made up of a plurality of essentially metal stripes ohmically interconnected by means of a bus bar stripe. Common materials for the front surface electrodes are silver, applied by a screen-printing process, or successive layers of titanium, palladium, and silver, applied by a metal evaporation process. In rare cases, aluminum has been used as the front surface electrode material, and the aluminum is alloyed with silicon. Known techniques for forming the aluminum stripes include evaporating aluminum onto the cell surface using a patterned mask, applying liquid aluminum to the cell surface and screen printing a mixture of aluminum and glass frit onto the cell surface in the stripe pattern. This last technique (screen printing), while convenient, suffers from the disadvantage of introducing high electrical resistance within the electrodes, since glass frit is a good electrical insulator. This disadvantage is particularly severe when the glass frit accumulates at the interface between the substrate material (typically silicon) and the aluminum alloy contact electrode. The actual location of the glass frit after formation of the electrode is dependent upon the details of the processing employed, and the process typically requires a temperature time spike to achieve effective alloying without excessively segregating the glass frit at the interface. This requirement adds complexity to the electrode formation process, which is undesirable. Typically, the non-contacted surfaces between the metal stripes on the cell are passivated using some type of thermal oxide, such as one of the oxides of silicon.
Electrons and holes can recombine at the illuminated surface of the semiconductive substrate or within the bulk of the material. Any such recombination of electrons and holes results in a reduction in solar cell conversion efficiency, and is thus highly undesirable. Recombination at the illuminated surface of the semiconductor substrate can occur in both the metal-electrode contacted region and the region not covered by the metal electrode.
Techniques have been explored for reducing the occurrence of electron-hole recombination at the illuminated semiconductor surface. For the metal-contacted region, the emitter of the p-n junction underlying the metal electrodes should be doped in a preferred range of 1.times.10.sup.19 cm.sup.-3 to 1.times.10.sup.21 cm.sup.-3, but workable devices can be made doped from 1.times.10.sup.19 cm.sup.-3 to 5.times.10.sup.21 cm.sup.-3 (termed at least moderately doped herein) and the junction should be relatively deep (e.g. .gtoreq.2 .mu.m) within the substrate. In contrast, for the non-contacted regions between the metal electrodes and under the passivated surface, the emitter portion of the p-n junction should be doped to a concentration of no greater than about 5.times.10.sup.18 cm.sup.-3 (termed lightly doped herein) and the junction should be relatively shallow (for example, about 0.2 .mu.m) within the substrate. Unfortunately, these two techniques pose conflicting requirements for doping the region adjacent the illumination surface of a solar cell. Although these opposing requirements can be implemented, the required processing is costly and complex, which results in a relatively high cost per cell fabricated and relatively low yield of acceptable functioning solar cells. For example, for high efficiency solar cells, the conflicting requirements of the two recombination techniques have been addressed by providing a second doped region under the electrode contact area. While this solution is effective, it requires two photolithography processing steps, two separate diffusion conditions for the emitter region, and an additional photolithography step in order to align the electrode grid pattern to the deeply diffused junction, all of which add substantial cost to the manufacturing process. Consequently, this solution is not consistent with the requirement of low cost for the fabrication of silicon solar cells for many applications.
In addition to the problems imposed by the electron-hole recombination phenomenon, another problem affecting the doping process involves silicon surface passivation. In particular, an effective silicon surface passivation requires a lightly-doped emitter since the observed surface recombination velocity increases with the surface doping concentration. Efforts to date to provide a low cost solar cell with effective reduction of electron-hole recombination have not met with success.